The augmentation of GNSS systems denotes a method used to enhance the reliability of such systems, in particular their availability, their accuracy and their integrity. Such a method is based on the integration, in the positioning calculation, of external information relating to different types of errors that can affect the GNSS signal. These errors relate, for example, to a desynchronization of the clocks of the satellite and of the receiver, to the position information concerning the satellites supplied by their ephemerides or even to the propagation delay of the signal through the ionosphere (ionospheric delay). An example of an augmentation system is the European EGNOS system which uses a plurality of ground stations with known positions, each permanently listening to the GNSS signals transmitted by the satellites and performing measurements on these signals. From the measurements made, notably concerning the time of reception of the signals, a message is transmitted to the users, via a relay satellite, to inform them as to the integrity of the positioning signal transmitted by the geolocation satellites. Such a message may also contain a correction to be applied to the measurement performed on the GNSS signal in order to determine positioning information. This correction is, for example, a time correction to be applied at the measured instant of reception of the GNSS signal. The application of this correction will enable the user to improve the accuracy of his or her positioning.
Thus, such a system makes it possible to augment the availability of the satellite positioning system since it generates suitable corrections that make it possible to use the GNSS signal even when it is affected by errors. In practice however, when the error measured by the ground stations exceeds a nominal operating threshold of the GNSS constellation, defined in the specifications of the GNSS system, the message transmitted simply indicates to the user that the corresponding positioning satellite is inoperative and that the user should perform his or her positioning by using the other satellites of the constellation, or even another positioning means if the satellite coverage is no longer sufficient.
The augmentation of a GNSS system can also be performed by transmission means other than satellite means. In particular, the correction and/or alert information can be transmitted through a cellular telephony network in the case where the GNSS receiver also includes reception means adapted to such a network. An example of such a system is the assistance system for GPS, known by the acronym A-GPS.
Hereinafter in the description, the term augmentation will be used to denote all the systems that make it possible to enhance the availability and the reliability of a satellite positioning system, whether these systems use satellite transmission means or, more generally, radio channel or cellular transmission means.
Augmentation thus has two functions: on the one hand, to warn the user of an operating problem affecting a satellite of the GNSS system so that the user will not use it, and also to provide the user with corrections enabling him or her to improve the measurements performed on a satellite of the GNSS system in its operating range.